1. Field of the Invention
The present invention relates to an automatic transmission controller for a vehicle and a method for controlling an automatic transmission system for a vehicle having different gear ratios that are established by selectively engaging a plurality of engaging elements.
2. Description of the Related Art
Automatic transmissions for a vehicle, which establish a plurality of speed steps having different gear ratios by selectively engaging a plurality of engaging elements, are used in a variety of vehicles. In recent years, in order to reduce fuel consumption and improve drivability, there has been a trend toward automatic transmissions for a vehicle with a higher number of speeds. Because of the increasing number of speeds in such automatic transmissions for a vehicle, multistep gear shifts, for example, from seventh gear to fourth gear, are often made. In such multistep gear shifts, it sometime occurs that two engaging elements are disengaged (two-element disengagement) and two engaging elements are engaged (two-element engagement), resulting in a gear shift that entails disengagement and re-engagement of four engagement elements. The Japanese Patent Application Publication No. JP-A-2003-106440 describes such gear shift control of an automatic transmission, wherein, for example, when shifting from sixth gear to third gear, the fifth gear, which is close in gear ratio to the sixth gear, is set as the intermediate gear, control being executed to make a gear shift to third gear via the fifth gear, which is the intermediate gear, thereby suppressing gear shift shock, for example.
In a gear shift entailing disengagement of two engaging elements and engagement of two engaging elements, with the art described in the Japanese Patent Application Publication No. JP-A-2003-106440, the gear shift time required to go from the starting gear to completion of the gear shift is increased, for example, by executing a gear shift to an intermediate gear. When the vehicle is running at a low speed, that is, when the rotating shaft of the transmission is rotating at a low speed, because there is a small difference in rotational speed between the synchronization speed and the input shaft of the transmission, the time for synchronization of the input shaft rotational speed to the synchronizing speed is shortened. As a result, in contrast to the usual situation, in which it is necessary to make the synchronization time and the time required for gear shifting approximately the same, there is a possibility that gear shift shock occurs at the time of a gear shift because there is insufficient time for gear shifting.